The hot box process is a high production method of producing cores and molds, used for casting metal pieces in foundry applications. The process involves the mixing of a latent acid catalyst, and a liquid thermosetting binder resin (e.g., a phenolic resole), with a quantity of foundry sand. The wetted sand mix is then blown into a heated pattern. The heat causes a curing mechanism to take place and a solid sand core or mold is obtained.
Typically, the catalyst/resin/sand mixture will become hard or gummy (non-flowable) when allowed to stand under ambient conditions for an extended period of time. The bench life of a sand mixture at ambient temperature can be defined as the time it takes for the mixture to become non-workable. Or put another way, the bench life can be defined as the maximum permissible time delay between mixing the binder components together with sand, and the production of acceptable products from the mixture. In most cases, a bench life of a few hours is sufficient. However, in some instances, a bench life greater than eight hours is required. For example, when the mixture is used to make molds and cores, a sand mixture may be required to remain unused in a storage hopper overnight. It is important that the sand mix not harden during this period because clean up would require additional effort, entail downtime, generate waste, and would mean a loss of efficiency. A means of extending the bench life of a hot box sand mixture to at least 24 hours would minimize these negative effects.
Current state of the art bench life additives, such as ammonia, have limited use as extender materials. Furthermore, ammonia has an associated odor problem. The use of effective carbonate materials such as calcium carbonate as bench life additives has the disadvantage of insolubility in either or both of the catalyst and the resin. Thus, an extra addition system is required when using these materials. Furthermore, carbonate materials can have a negative effect on the tensile strengths of the cores produced.
We have now found that the use of an alkali metal salt of a polybasic acid as an additive to a hot box sand mixture can extend the bench life of the coated sand mixture. A bench life extender of this type may allow a production batch of the resin coated sand to remain unused in a hopper for extended periods and still remain workable.
According to one embodiment of the invention, a bench life extension additive, such as tripotassium citrate or dipotassium phosphate, can be added to the sand as a solid before the resin and catalyst are added, at a level of 0.01 to 0.1% based on sand weight, in which case three components are added to the sand. The bench life extension additive, the resin and the catalyst can be added to the sand in any order. Alternately, the additive can be formulated into the resin or catalyst, in which case only two components (the catalyst component and the resin component) need be added to the sand.
The bench life extension materials of the invention have the advantage of being soluble in the catalyst, and they are low in odor. Thus, the use of these materials would not increase production steps, and they are generally compatible with the components and equipment used to produce hot box foundry cores and molds, while maintaining the desirable properties of the cured cores and molds.